1. Technical Field
The disclosure relates to a multimode (MM) fiber configured with the geometry allowing for the excitation of substantially a fundamental mode at the input of the MM fiber and for guiding the excited fundamental mode without coupling thereof with high order modes.
2. Known Art Discussion
Numerous applications of fiber lasers are in need of a high-power, high-quality beam. Fiber lasers utilizing SM active fibers are limited in power due to the onset of optical nonlinearities. One common solution is the use a MM active fiber capable of supporting a few high-order modes (HOM) but configured to prevent the excitation and amplification of these HOMs.
Yet the power scaling of such MM fibers is also somewhat limited by the presence of nonlinearities including, but not limited to, Stimulated Raman Scattering (SRS). Perhaps one the most efficient practical approaches, leading to a relatively high optical nonlinearity threshold, is to decrease the power density inside the core of a MM fiber by increasing the core diameter, decreasing a numerical aperture and also decreasing the effective length of non-linear interaction. Unfortunately, this geometry is not easily attainable for the following reasons. First, the increase of the core diameter results in the increased number of HOMs which can be easily excited that detrimentally affects the quality of the output beam. Second, the manufacturing of high quality fibers with truly very low Δn is highly challenging. Third, such fibers are sensitive to bending loads.
One of the techniques known to an artisan in the fiber laser art includes configuring the core of MM fiber with a dopant profile occupying the core's central region, as will be discussed in below. The centrally doped region effectively minimizes the excitation of peripheral non-zero azimuthal number HOMs, such as LP 11, which is traditionally considered as either originated at the input of MM fibers or as a result of mode coupling. However, the centrally doped region stimulates the excitation of centrally symmetrical HOMs with zero azimuthal number, such as LP02, at the input end of the MM fiber. The excited central modes, like any other excited HOMs, tend to extract the power from a fundamental mode and, thus, represent a serious problem.
A need, therefore, exists for a MM fiber configured with an axial cross section providing most favorable conditions for exciting substantially only a fundamental mode.
Another need exists for a MM fiber with a core configured with a refractive index which provides for the increased effective area of the excited fundamental mode and higher thresholds for nonlinearities that allow the MM fiber to output radiation on the order of tens of kW substantially concentrated in the fundamental mode.
Still another need exists for the MM fiber core with a dopant profile configured to gain guide substantially a fundamental mode LP01 without coupling thereof with central symmetrical modes, such as LP02.
Yet a further need exists for a high-power fiber laser system configured with the above described MM fiber amplifier which is configured to provide a substantial gain differential between fundamental and high order modes.